Piezoelectric crystal apparatus



April 29, 1941. J. M. WOLFSKPLL PIEZOELE'CTRIC CRYSTAL APPARATUS FiledApril 22, 1940 Enventor (T01 212 if. 7VaZfs%1/ZZ.

Jttome g the electrodes.

Patented Apr. 29, 1941 PIEZOELECTRIC CRYSTAL APPARATUS John M.Wolfskill, Erie, Pa, assignor to Bliley Electric Company, Erie, Pa., a,partnership composed of F. Dawson Bliley and Charles Oollman ApplicationApril 22, 1940, Serial No. 331,050

Claims.

This invention relates to piezoelectric crystal apparatus, and moreparticularly to a method for mounting piezoelectric crystals.

One of the objects of the present invention is to provide a means ofrigidly clamping the crystal to one of the electrodes to prevent anylateral movement of the crystal due to vibration or shock, and at thesame time provide adjustable means for controlling the second electrodeof the holder to permit the crystal frequency to be varied by varyingthe air gap between the adjustable electrode and the correspondingcrystal surface.

Another object is to provide a crystal mounting which gives improvedperformance of the crystal over a wide temperature range.

Another object is to provide resilient clamping means for the crystal,so that the pressure is substantially evenly distributed over theclamping areas.

Another object is to provide clamping means which are independent of thesecond electrode of the piezoelectric crystal mounting.

Still another object is to provide clamping means for high or ultra highfrequency crystals whereby the performance is greatly improved becausethe clamping'means is remote from the active area of the crystal.

Heretoiore a crystal was generally mounted between two ordinary planefaced electrodes, with spring means for holding the electrodes incontact with the crystal. It has been found that greatly improvedperformance may be obtained from acrystal in certain parts of thefrequency spectrum-by designing the electrode so as to give minimumcapacity over the area of the crystal which is oscillating. I accomplishthis in the present invention by cutting away a portion of one or bothof the electrodes of the piezoelectric crystal and by leaving arelatively small substantially flat button shaped electrode portionpreferably adjacent the center of the corresponding piezoelectriccrystal face. This small button shaped electrode is adapted toconcentrate the electric field substantially at the central portion ofthe piezoelectric crystal and at the same time decreases the detrimentalelectrostatic capacity of the holder present between the outer areas ofAt the same time small projections or areas of at least one of theelectrodes engagethe crystaladjacent to the edges thereof to clampthecrystal into the holder thus permitting the small button shaped portionof the electrode to be slightly spaced from the surface of thepiezoelectric crystal to prevent abrasion.

. detail view of the bottom electrode of the crystal holder shown inFig. 1; Fig. 3 is a cross-section of another electrode adapted for usein the holder shown in Fig. 1, and Fig. 4 is a detail view of one of thecrystal element clamping strip fasteners.

Referring to the drawing in detail, reference numeral ID designates apiezoelectric crystal element of quartz, tourmaline and the like,- cutrelatively thin with two major faces substantially parallel to eachother or slightly convexed. The crystal element In is positioned on theelectrode H of conducting material, such as, brass, aluminum, stainlesssteel and the like, so as to ride over the cavity or recess l2 formed insaid electrode. This is illustrated more clearly in Fig. 2 where thecrystal element II! is shown in broken lines. The recess or cavity l2does not extend-entirely through the electrode but only partially so asto reduce the electrostatic capacity between it and the crystal element.In some cases it is desirable to provide a small buttonshaped electrodesubstantially tothe center of the cavity l 2 as shown in Fig. 3, toconcentrate the electricfield substantially on the center of the crystalelement. This button Ila also is slightly spaced from the adjacentcrystal element surface in order to eliminate abrasion between theelectrode and the crystal element. If abrasion is not eliminated finedust will be produced and this will seriously impair the operation orthe crystal element if it is not periodicallyremoved.

The corners of the crystal element II] are resiliently clamped by thestrips l3 and IE to the more or less triangularly shaped areas llbformed around the cavity l2. These strips l3 and I5 may be made ofvarious materials, such as, brass, bronze, Bakelite and the like and, ofcourse, are not of such weight or strength as to break or fracture thecrystal element by their clamping action. Suitable threaded holes areprovided in the electrode l I for receiving the screws I4 and 16 to holdthe clamping strips l3 and 15, respectively, and where desired coilsprings Ma as illustrated in Fig. 4 maybe used on, each of the screws l4and I6 particularly where the strips [3 and I5 are made of relativelythick material which itself does not have sufflcient resilience.

A circular electrode I1 is adjustably supported by the threaded memberl8 over the top of crystal element I 0. This electrode also may be ofbrass, stainless steel or the like and is preferably flat on the surfaceadjacent the crystal. The member I8 is relatively tightly threaded intothe bar I9 so that the air-gap between the electrode l1 and the crystalelement may be adjusted and the frequency of the crystal element variedwhile the crystal elementis oscillating,byinserting a screw driver orsimilar tool, preferably made of insulating material, into the groove ofthe threaded member I8 and rotating this member until the desiredelectrode air gap is obtained. The bar I! may be of electricallyconducting material to facilitate connecting the electrode I! into anelectrical circuit. However the pillars 2| are of ceramic material, suchas, Isolantite, Alsimag, synthetic porcelain and like materialpreferably such as may be relatively easily lapped so that the top andbottom surfaces may be made parallel. This is important becauseotherwise the bar l9 would not be positioned correctly with respect tothe top surface of the crystal so that the air-gap of the electrode I!could be accurately adjusted. The pillars or bushings 2| are attached tothe base H by suitable screws and to the bar I! by screws 20.

The advantage of this piezoelectric crystal holder is that it givesexcellent frequency stability since it improves performance of thepiezoelectric crystal by tending to eliminate what is known as power orfrequency jumps encountered as the temperature of the crystal and holderis varied over a substantial range. This is a rather common fault ofpresent day piezoelectric crystal units since in most applications thetemperature of the units is varied over a considerable range duringwhich the crystal element must give satisfactory performance.

Considerable difliculty has been encountered in preventing objectionablevariations in power output from the crystal controlled oscillator whenpiezoelectric crystal holders of types generally known in the art wereemployed. While the frequency variation of the piezoelectric crystalover wide temperature ranges may be kept within certain limits byemploying low drift type piezoelectric crystal cuts, the problem ofmaintaining the power output from the piezoelectric crystal oscillatorfrom varying beyond a certain percentage of the maximum output reachedthroughout the temperature range was more difficult to solve. It wasfound that if a low drift type of crystal element were placed betweenordinary electrodes and the temperature varied from to 60 degreescentigrade that the output power fluctuated over wide ranges. This waseliminated by continuing the cut and try grinding until all of theundesired couplings were practically eliminated and the crystal elementbecame stable over the temperature range. However in the presentinvention this undesired power output fluctuation is practically removedby employing the peizoelectric crystal element between the specialelectrodes which clamp the piezoelectric crystal element securely at thecorners and concentrate the electric field in the active central portionof the element. This has the tendency of discouraging any unwanted modesof oscillation which may be coupled into the length and width modes. Itis in fact possible to take piezoelectric crystal elements which mayhave as much as a 65% change in power output through the 0 to 60 degreescentigrade temperature range and obtain practically complete stabilitythroughout this range by employing a holder made in accordance with thisinvention.

When high frequency piezoelectric crystals are employed a very smallair-gap is used to reduce the damping and keep the activity of thepiezoelectric crystal as high as possible. It is desirable when usinghigh frequency crystals to at times use an air-gap as low as several tenthousandths of an inch and this means that the active electrode andcrystal electrode faces have to be parallel to a tolerance even lessthan this. Another advantage of employing a slight air-gap between theworking surfaces of the piezoelectric crystal element and the electrodesis that abrasion can not take place between these surfaces if thecrystal should shift its position.

In the ultra high frequency region it is desirable to reduce theelectrostatic capacity of the piezoelectric crystal holder to the veryminimum in order to reduce the shunting effect of this capacity to theultra high frequency currents produced. For this reason in addition tothe other reasons given the button type of electrode is employed.However it was found practically impossible to maintain a fixed positionof a small button electrode relative to the crystal element surface andconsequently the electrode employed in the present invention consists ofa small button electrode formed by cutting away the sections of theelectrode between the button and the clamping corners. Cutting away orrecessing a portion of the electrode reduced the damping adjacent to theexcited area of the crystal element and also reduced the total capacityof the holder to a minimum. It also permitted clamping the piezoelectriccrystal element between surfaces integral with the electrodes.

While I have described this invention in detail with respect to certainembodiments thereof it is of course understood that I do not desire tolimit it to the exact details shown except in so far as they may bedefined by the claims.

What I claim is as follows:

1. Piezoelectric crystal apparatus, comprising: a relatively thinpiezoelectric crystal having a pair of major faces substantiallyparallel to each other and adapted to oscillate on radio frequenciesapproaching the ultra high frequencies, a pair of electrodes, one foreach of the aforesaid major faces of said piezoelectric crystal, one ofsaid electrodes having portions thereof engaging surfaces along the edgeof the adjacent major face of said crystal, said last mentionedelectrode having a portion of the face adjacent to the crystal cut awayso that the remaining portion not cut away forms a substantiallycentrally disposed projection extending almost to a substantiallycentrally disposed active portion of the crystal, a pair of strapmembers extending longitudinally across edge portions of said crystalelement for resiliently clamping said crystal element to said lastmentioned electrode, means for supporting the other of said electrodes,and means for adjusting said last electrode to or away from thecorresponding crystal face.

2. Piezoelectric crystal apparatus as set forth in claim 1 furthercharacterised in that said means for supporting the other of saidelectrodes consists of spacing means having the top and bottom surfacesthereof lapped substantially to parallelism.

3. Piezoelectric crystal apparatus as set forth in claim 1 furthercharacterised in that said means for supporting the other of saidelectrodes consists of a plurality of ceramic spacing pillars having thetop and bottom surfaces thereof lapped substantially to parallelism.

4. Piezoelectric crystal apparatus as set forth in claim 1 furthercharacterised in that said means for supporting the other of saidelectrodes consists of spacing means having the top and bottom surfacesthereof lapped substantially to 10 parallelism, and a member attached tothe top of said spacing means, said means for adjusting

